Flying under the radar: How livestock antibiotics influence house fly development

On a global scale, antibiotics are administered extensively to livestock for therapeutic, prophylactic, and/or growth-enhancing purposes, especially in animals kept in confined animal feeding operations (CAFOs) like feedlots. Antibiotics are excreted from large numbers of animals in waste (urine and manure) which accumulates in these operations. CAFO waste rich in antibiotics and other chemicals used to manage disease and pests in the operation also serve as a developmental substrate for house flies (Musca domestica L). The impact that high levels of antibiotic residues in manure have on house fly biology, particularly larval development, has not been well described. In this study, we determined the effects of two antibiotics commonly used in beef cattle production (tetracycline and enrofloxacin) on house fly larval development. Both antibiotics were administered to the larval substrate (separately and in combination) at low, medium, and high concentrations: 43.6, 436, and 4360mg/kg for tetracycline; 10.5, 105, and 1050mg/kg for enrofloxacin. Dose dependent effects on house fly larval development were observed including slowed larval time to pupation, lower pupation rate, reduced pupal mass, and reduced eclosion rate. Our results indicate that antibiotic residues in animal production systems affect house fly larval development. To confirm this reduction in fitness was due to antibiotics, bacterial analyses of the larval growth media were conducted. Bacterial communities contained the same isolates regardless of treatment or developmental time. Future studies will explore whether antibiotics select for flies tolerant to chemical stressors, potentially impacting pest management.